The invention relates to a beam force measuring system for cyclically operating machines such as injection molding machines.
The invention will be described principally to production machines in the plastics industry, especially to cyclically operating injection molding machines. It can, however, be applied also to presses, impact forming machines and in general, equipment where variable forces with large screw connections have to be monitored. The measuring system might thus be of importance in chemical process engineering.
To explain the invention in more detail, an injection molding machine will be taken as an example.
For monitoring a uniform injection process in the mass production of plastic parts, the pressure behavior in the injection mold is decisively important. The pressure curve in the mold is measurable via the change in the pressing together of the two mold halves in the usually four beams or tie bars of the machine. If the force pattern in the beams is known, a complete in-mold pressure curve can be derived automatically. In many cases, this variable can be used directly to control the injection screw. For intricate precision moldings where the pressure build-up must be followed at several points, special pressure sensors are fitted in the actual mold.
Various measuring methods are employed to detect the patterns of the clamping forces in the tie bars. All techniques for measuring on beam surfaces, like attaching strain gauges, are precluded because they obstruct the free movement of the mold carriers. Consequently, force measuring techniques have been developed which gauge the strain in the end bores of the tie bars. Their measuring outputs are located at the control ends of the tie bars.
In practice, long boreholes are needed in order to place the strain sensors properly in the zones of uniformly distributed tensile stress. Badly placed strain sensors give false measurements.
In many applications therefore, there is a need for a solution which does not require drilling the highly loaded tie bars. According to the invention, a thin disk sensor having a very low overall height is placed underneath the tie bar tightening nut. No mechanical alterations are necessary on the force transfer parts of the tie bars, so that the invention is equally suitable for original equipment and retrofitting.
According to the state of the art, with strain sensors in the bore, an absolute accuracy of the beam force around .+-.3% can be attained. With the thin disk sensor according to the present invention, the works-calibrated sensor may be evaluated direct, enabling accuracies of .+-.1%. A recalibration in the fitted state may point to jamming of the tie bars in the guiding frame, which must be corrected.
The invention provides new possibilities for monitoring the quality of the moldings and the machine functioning, without entailing mechanical alterations on important force-conducting parts. The invention is therefore suitable not only for new machines but especially for retrofitting on machines already in operation.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.